1. Field of the Invention
The present invention relates to a wavelength-multiplexed light amplifying apparatus, an optical amplifier, and further to an optical add-and-drop apparatus (optical ADM:Optical Add-Drop Multiplexer) using a wavelength-multiplexed light amplifying basic unit.
2. Description of the Related Art
In recent years, the improvement of the transmission technology using an optical fiber has developed enterprises related to point-to-point wavelength division multiplexing (WDM), where the technical development for a photonic network has taken place extensively. This photonic network signifies a network which uses optical wavelengths as identification information for multiplexing and non-multiplexing. As one of the features the photonic network has, there is an optical add-and-drop function. The dropping function of an optical add-and-drop unit in this photonic network is realizable by a broadcast function. This broadcast function means a function of distributing the directions multiplexed signals advance in, but not different from a function of demultiplexing an optical signal power. Referring to FIG. 18, a description will be given hereinbelow of the broadcast function according to a conventional technique.
FIG. 18 is an illustration of one example of configuration of an optical add-and-drop multiplexer. For instance, let it be assumed that, when an optical signal comprising a plurality of wavelengths (ch1 to ch64) multiplexed is transmitted from a city A positioned on the left side in FIG. 18 to a city B positioned on the right side in the same illustration, in a city C lying between these cities A and B, of these wavelengths, ch1 to ch32 are dropped, whereas different ch1 to ch32 are instead added thereto. In this instance, ch33 to ch64 are passed through. A wavelength-multiplexed optical signal coming from the left side in FIG. 18 is amplified in an optical amplifier 70a while another optical signal coming through an adding section 70d is then added thereto in an AOTF (AcoustoOptical Tunable Filter) 70b. Furthermore, a portion of the first-mentioned optical signal is dropped and inputted to a branching section 71, while the second-mentioned optical signal is amplified in an optical amplifier 70c and then transmitted to the right side in FIG. 18. This AOTF 70b is a convenient device while it is difficult to make it. AOTF 70b can drop ch1 to ch32 and newly add ch1 to ch32. Herein, the channel number is equivalent to the assigned wavelength. For example, when ch1 is dropped in the AOTF 70b, ch1 becomes totally empty so that another information can be assigned to ch1 and added thereto. On the other hand, after the optical signal dropped in the AOTF 70b is amplified in an EDF optical amplifier (Erbium-doped Fiber Optical Amplifier) 71a, its optical signal power is divided or split in an optical coupler 71b and subsequently inputted to tunable filters 72 so that the wavelength-multiplexed optical signal appears at each port. In this case, the reason for the amplification in the EDF optical amplifier 71a is that, for example, if an optical signal is divided into 1000, its power reduces to 1/1000. That is, a need for the amplification exists for maintaining the original power. In addition, the optical signal is distributed through the use of a splitter not having a wavelength characteristic or an AWG (Arrayed Waveguide Grating) having a wavelength characteristic. That is, a component for realizing this optical add-and-drop function utilizes a property of changing its advancing direction in accordance with an optical wavelength. When a wavelength-multiplexed light is distributed in relation to each desired wavelength in this way, since that distribution is made after it is amplified up to an extremely high output level in an optical amplifying system, the loss of the optical signal in the latter section of the EDF optical amplifier 71a has great influence on the efficiency. This efficiency signifies the efficiency of conversion from an excitation optical power into a signal optical power.
In FIG. 18, let it be assumed that the dropped 32ch-wavelength-multiplexed signal is distributed or branched into 16 ports. Additionally, let it be assumed that 0 dBm per Ich (which will be referred to hereinafter as 0 dBm/ch) is kept as an optical signal quality at each branch (division) port. In this case, for maintaining 0 dBm/ch at the optical signal quality at each branch port, an output power at each branch port requires 15 dBm (32 mW for 32 ch), and a value forming the sum of 15 dBm and a theoretical limit value on the division into 16 becomes necessary at an output terminal of the EDF optical amplifier 71a. This theoretical limit value signifies the value of the original optical signal needed for the optical signal to have a given quality after the division. As well known, when the original optical signal is divided into two, each of the optical signals after the division results in the loss of 3 dB as compared with the original value, and when being divided into 16, each of the optical signals after the division suffers a loss of 12 dB as compared with the original value. Accordingly, when one optical signal is divided into 16, the original optical signal is required to have a value larger by at least 12 dB than the power value of each of the divided optical signals. In other words, in the case that one optical signal is divided into 16, the theoretical limit value of the output power of the EDF optical amplifier 71a becomes 12 dB, and in this instance, the needed value at the output terminal of the EDF optical amplifier 71a comes to 15 dBm+12 dBm=27 dBm. Additionally, taking into consideration an excess loss of the distribution (branch) optical coupler 71b, the output power of (27+xcex1) dBm becomes necessary at the output terminal of the EDF optical amplifier 71a. In the following description, this value xcex1 will be taken as 2 dB, for example. That is, the output power of 29 dBm becomes necessary at the output terminal of the EDF optical amplifier 71a. 
On the other hand, looking at a system of input to this EDF optical amplifier 71a, since 0 dBm/ch is needed while 32 ch is dropped, the total input power to the EDF optical amplifier 71a comes to 15 dBm. Besides, this EDF optical amplifier 71a is made up of optical parts such as an optical isolator and an optical coupler, which causes a loss of approximately 2 dB. In consequence, a large output of 31 dBm(=27+2+2=1260 mW) develops at the output terminal of an EDFA (Erbium-Doped Fiber Amplifier) (not shown) in the EDF optical amplifier 71a, while the output terminal of the EDF optical amplifier 71a produces 29 dBm(=790 mW). Additionally, assuming that the conversion efficiency from an excitation light to a signal light in the EDF optical amplifier 71a is 50%, the required excitation power reaches 1580 mW.
Thus, the operating conditions of this optical amplifying section 71a are as follows.
1) input power: 15 dBm (0 dBm/ch, 32 waves)
2) output power: 29 dBm (31 dBm at the output terminal of EDFA)
3) gain: 14 dB
4) required excitation power: 1580 mW
From this, it is found that a large signal optical power of 1260 mWxc3x970.37=470 mW (1260 mW=31 dBm) disappears between the output terminal of the EDF optical amplifier 71a and the output terminal of the optical amplifying section 71a. That is, since an excess loss (0.37; corresponding to xe2x88x922 dB) occurs at the time when the signal power rises, the useless signal power increases. Additionally, even if the divisions is small in number, because a high-output optical amplifier becomes necessary at the initial introduction, the initial investment becomes higher.
For this reason, there is a need to realize the broadcast function efficiently. That is, if a broadcast system with higher efficiency is realized, a signal with a desired wavelength becomes selectable in a manner that an optical filter for performing wavelength selection at every port is placed at a spot subsequent to the point of distribution of the optical signal, and the flexible distribution becomes feasible. Besides, if realized, this broadcast function is also applicable to a distribution system such as an optical subscriber system.
In addition, Japanese Laid-Open (Kokai) HEI 3-123323 (which will be referred to hereinafter as a publication) discloses a technique on a fiber coupler for preventing the reduction of the optical intensity, occurring whenever the division takes place, to considerably increase the dividable hierarchies. FIG. 19 shows an arrangement of this fiber coupler. In the arrangement shown in FIG. 19, fiber couplers are combined to provide three division hierarchies, where a signal light S0 inputted to an input section 3 is introduced into a branching section 2 while an excitation light P inputted to another input section 3 is also led to the branching section 2 so that they are distributed to an output sides 4 of an optical fiber and then transmitted to amplification fibers 5. The signal light amplified in the amplification fiber 5 is subsequently distributed through a second branching section 12 to second optical fibers 13 and further distributed through third branching sections 14 to third optical fibers 15, thus outputting signal lights S1 to S8 therefrom.
However, the technique disclosed by this publication relates to light with a single wavelength, but the publication does not refer to a technique on wavelength-multiplexed light at all.
The present invention has been developed with a view to eliminating the above-mentioned problems, and it is therefore an object of the invention to provide a wavelength-multiplexed light amplifying apparatus and an optical amplifier to be connected to branch ports of an add-and-drop apparatus for adding a wavelength multiplexed optical signal and for dropping and outputting a wavelength multiplexed optical signal, or a wavelength-multiplexed light amplifying apparatus and an optical amplifier to be used for an mxc3x97n matrix optical switch in an optical cross-connect unit, where basic amplifying units to which 1:1 optical couplers and EDFAs are coupled alternately are connected in a multistage fashion to distribute a power loss of an optical signal for accomplishing a high-efficiency amplification, to enable the expansion of the branch ports on the in-service, to render an optical isolator unnecessary or reduce the required isolation value for stabilizing the system, and to implement control so that the gains at all the ports are maintained constant, and further to provide an optical add-and-drop apparatus using a wavelength-multiplexed light amplifying basic unit.
For this purpose, in accordance with this invention, there is provided a wavelength-multiplexed light amplifying apparatus comprising a plurality of wavelength-multiplexed light amplifying units, with a signal input port of an optical branching section in one wavelength-multiplexed light amplifying unit connected to an output side of an optical amplifying section in the preceding wavelength-multiplexed light amplifying unit, and an optical feedback loop system.
Thus, as compared with the case of amplifying a wavelength-multiplexed optical signal as a whole, the optical signal power reduces considerably, which presents a higher-efficiency characteristic. In addition, since each of the first to third wavelength-multiplexed light amplifying basic units is composed of the optical branching section having the two input ports and the two output ports for outputting a wavelength-multiplexed optical signal through the two output ports and the optical amplifying sections connected to the two output ports of this optical branching section, the amplification mediums are disposed dispersedly in an optical distribution system, which permits the realization of a stable optical amplifier without relying on an optical isolator. Still additionally, since the optical amplifying sections and the optical branching sections are connected alternately to each other, the effective gain in the amplifier lowers to produce a reflection resistance, and if the gain of the optical amplifying section is set at a lower value than the division (branch) loss, the effective gain becomes minus, which eliminates the need for each optical amplifying section being interposed between optical isolators and, hence, simplifies the system. Moreover, the excess loss of a system including the optical amplifier and the distribution system is reducible so that the unstable operation due to the reflection is eliminable without using an optical isolator.
In this configuration, it is also appropriate that an auxiliary excitation light source is connected to the other input port of at least one of the plurality of wavelength-multiplexed light amplifying basic units, or that an isolator is connected to the signal input port of the first wavelength-multiplexed light amplifying basic unit in at least one of the plurality of wavelength-multiplexed light amplifying units.
Furthermore, it is also appropriate that an auxiliary excitation light source is connected to the other input port of the aforesaid wavelength-multiplexed light amplifying basic unit, or that an isolator is connected to the signal input port of the aforesaid wavelength-multiplexed light amplifying basic unit.
Still furthermore, it is also appropriate that an auxiliary excitation light source is connected to the other input port in at least one of the plurality of wavelength-multiplexed light amplifying basic units, or that an isolator is connected to the signal input port in at least one of the plurality of wavelength-multiplexed light amplifying basic units.
In this case, it is possible to strengthen the excitation optical power when needed, and since the excitation light source can be distributed equally through the use of the optical branching section (1:1 optical coupler), the excitation optical power can be supplied equally through a simple configuration to each of the optical amplifying sections, which eliminates the need for the use of multiplexing units for adding or multiplexing an excitation light and a signal light, or reduces the required number of multiplexing units.
In addition, in addition to avoiding the unstable operation due to the reflecting and returning light, the reverse ASE gathering from the output side to the input side is shut off, thereby preventing the saturation of the optical amplifier due to the reverse ASE.
Moreover, it is also acceptable that, in at least two of the plurality of wavelength-multiplexed light amplifying units, the output side of the optical amplifying section of each of the second and third wavelength-multiplexed light amplifying basic units in the preceding wavelength-multiplexed light amplifying unit are connected through an oblique (skew) connector to the input side of the signal input port of the first wavelength-multiplexed light amplifying basic unit in the subsequent-stage wavelength-multiplexed light amplifying unit.
Furthermore, it is also acceptable that, in at least two of the plurality of wavelength-multiplexed light amplifying basic units, the output side of the optical amplifying section of the preceding wavelength-multiplexed light amplifying basic unit is connected through an oblique connector to the input side of the signal input port in the subsequent-stage wavelength-multiplexed light amplifying basic unit.
In this case, light reflection does not occur on the connector surface, so that useless light amplification is preventable.
Still furthermore, it is also possible that an input monitor is provided to detect the input to the fourth wavelength-multiplexed light amplifying basic unit, and the gain control means includes a control section for producing a gain control signal on the basis of the output information and the detection result by the input monitor and for supplying the gain control signal to an excitation light source provided on the input side of the gain equalizer for controlling an excitation state of the excitation light source.
In this case, the control of the wavelength characteristic of the gain at all the branch ports becomes feasible, and the control of the wavelength characteristic of the gain becomes possible at all the ports in a manner that feedback control is implemented for the forefront excitation light source.
Furthermore, in accordance with this invention, there is provided a wavelength-multiplexed light amplifying apparatus comprising a plurality of wavelength-multiplexed light amplifying basic units, with the wavelength-multiplexed light amplifying basic units being connected in a multistage fashion in a manner that the signal input port of the optical branching section is connected to an output side of the optical amplifying section of the preceding wavelength-multiplexed light amplifying basic unit and an optical feedback loop system.
Thus, if the user wants the extension of the service, the extension in service becomes easily feasible. That is, by newly and additionally buying this wavelength-multiplexed light amplifying basic unit, the user can easily increase the number of divisions. This extension can be performed without interrupting the current service. In addition, if the forefront excitation light source has a power sufficient to excite the rearmost wavelength-multiplexed light amplifying basic unit, there is no need to place an excitation light source in the intermediate wavelength-multiplexed light amplifying basic units. Still additionally, an excitation light source can be installed in the intermediate wavelength-multiplexed light amplifying basic units when needed, and excitation light sources can be connected to all the wavelength-multiplexed light amplifying basic units, thus achieving flexible excitation. Besides, the respective elements are constructed with optical parts having the same characteristic, and this suits the mass production, and offers excellent expansion.
Furthermore, in this configuration, it is also possible that an input monitor is provided to detect the input to the forefront unit of the plurality of wavelength-multiplexed light amplifying basic units arranged in multistage fashion, and the gain control means includes a control section for producing a gain control signal on the basis of the output information and the detection result by the input monitor and for supplying the gain control signal to an excitation light source provided on the input side of the gain equalizer for controlling an exciting state of the excitation light source.
In this case, the control of the wavelength characteristic of the gain at all the branch ports becomes feasible, and the control of the wavelength characteristic of the gain becomes possible at all the ports in a manner that feedback control is implemented for the forefront excitation light source.
Still furthermore, in accordance with this invention, there is provided a wavelength-multiplexed light amplifying apparatus comprising a wavelength-multiplexed light amplifying basic unit and an optical feedback loop system.
Thus, this system can be offered as simple equipment to the users, while the users can easily increase the number of divisions without interrupting the current service.
In this configuration, it is also possible that an input monitor is provided to detect the input to the wavelength-multiplexed light amplifying basic unit arranged in multistage fashion, and the gain control means includes a control section for producing a gain control signal on the basis of the output information and the detection result by the input monitor and for supplying the gain control signal to an excitation light source provided on the input side of the gain equalizer for controlling an exciting state of the excitation light source.
Thus, the control of the wavelength characteristic of the gain at all the branch ports becomes feasible, and the control of the wavelength characteristic of the gain becomes possible at all the ports in a manner that feedback control is implemented for the forefront excitation light source.
In this case, it is possible that the optical branching section of the aforesaid wavelength-multiplexed light amplifying basic unit is constructed as a 1:1 optical coupler and the optical amplifying section of the aforesaid wavelength-multiplexed light amplifying basic unit is constructed to have a gain equivalent to a division loss in the optical branching section or a gain lower than the division loss, or that the optical branching section, the optical amplifying section and a connecting section between the optical branching section and optical amplifying section in the aforesaid wavelength-multiplexed light amplifying basic unit are constructed as an optical waveguide made from a glass or a semiconductor.
Thus, the division and the amplification are alternately conducted when viewed from the output side, and each of the optical amplifying sections can maintain the theoretical limit value (for example, 15 dBm) as long as it performs the amplification at a gain (for example, 18 dB) with a level or magnitude capable of compensating for the power loss (for example 3 dB) occurring at the division into two, thereby presenting high-efficiency expansion. Additionally, a stable optical amplifier becomes realizable, and there occurs no situation where the optical power strengthens locally, which offers a higher-efficiency characteristic.
In addition, it is also possible that the aforesaid gain control means is equipped with a filter for selecting a desired-wavelength optical signal from the output information and an optical attenuator for attenuating the desired-wavelength optical signal extracted through the filter for the laser resonance and further for supplying the attenuated optical signal to the input side of the gain equalizer. This filter can also be constructed as a filter for selecting an optical signal with a wavelength being out of a wavelength band needed for transmission.
Thus, the port to be feedbacked is only one in number, and the forefront excitation light source can vary the excitation light to maintain the gain of each of the ports constant, so that the gains of all the ports are maintainable constant with a simple construction.
Still furthermore, in accordance with this invention, there is provided a wavelength-multiplexed light amplifying apparatus made to be connected to a branching port of a multiplexing/branching unit which receives a wavelength-multiplexed optical signal composed from optical signals of M kinds of wavelengths (M signifies a natural number) and a wavelength-multiplexed optical signal composed from optical signals of N kinds of wavelengths and further which branches them into a wavelength-multiplexed optical signal composed from optical signals of (M+Nxe2x88x92L) kinds of wavelengths (N and L signify a natural number) and a wavelength-multiplexed optical signal composed from optical signals of L kinds of wavelengths, the wavelength-multiplexed light amplifying apparatus comprising a plurality of wavelength-multiplexed light amplifying basic units each including an optical branching section for receiving the wavelength-multiplexed optical signal, and optical amplifying sections respectively connected to the two output ports of the optical branching section, with the wavelength-multiplexed light amplifying basic units being connected in a multistage fashion in a manner that the signal input port of the optical branching section in one wavelength-multiplexed basic unit is connected to an output side of the optical amplifying section of another preceding wavelength-multiplexed light amplifying basic unit, and an excitation light source connected to the other input port of the optical branching section of the forefront unit of the plurality of wavelength-multiplexed light amplifying basic units.
Thus, a broadcast function is easily realizable, and the extension of the branch ports on the in-service becomes possible.
Moreover, in accordance with this invention, there is provided a wavelength-multiplexed light amplifying apparatus for use in an mxc3x97n matrix optical switch (m and n signify a natural number) in an optical cross-connect unit, comprising a plurality of wavelength-multiplexed light amplifying basic units each including an optical branching section for receiving a wavelength-multiplexed optical signal and optical amplifying sections respectively connected to the two output ports of the optical branching section, with the wavelength-multiplexed light amplifying basic units being connected in a multistage fashion in a manner that the signal input port of the optical branching section in one wavelength-multiplexed basic unit is connected to an output side of the optical amplifying section of another preceding wavelength-multiplexed light amplifying basic unit, and an excitation light source connected to the other input port of the optical branching section of the forefront unit of the plurality of wavelength-multiplexed light amplifying basic units.
Thus, this system is applicable to a distribution system such as an optical subscriber system, and the expansion on the in-service is easily feasible.
In addition, in accordance with this invention, there is provided an optical add-and-drop apparatus using wavelength-multiplexed light amplifying basic units, comprising a first-stage unit for distributing a wavelength-multiplexed optical signal composed from optical signals of X kinds of wavelengths, a first broadcast unit connected to an output side of the first-stage unit for broadcasting the wavelength-multiplexed optical signal outputted from the first-stage unit, a second broadcast unit connected to the output side of the first-stage unit for broadcasting the wavelength-multiplexed optical signal outputted from the first-stage unit and for dropping an optical signal, an optical carrier signal outputting section for outputting an optical carrier signal and for adding an optical signal and a selecting section connected to an output side of the first broadcast unit and an output side of the optical carrier signal outputting section for selectively outputting the wavelength-multiplexed optical signal from the first broadcast unit and the optical carrier signal from the optical carrier signal outputting section, wherein the first-stage unit is constructed as a first-stage wavelength-multiplexed light amplifying basic unit composed of an optical branching section for receiving the wavelength-multiplexed optical signal, composed from the optical signals of the X kinds of wavelengths and optical amplifying sections respectively connected to the two output ports of the optical branching section, and the first broadcast unit includes a plurality of multiplexing wavelength-multiplexed light amplifying basic units each identical in configuration to the first-stage wavelength-multiplexed light amplifying basic unit, with these multiplexing wavelength-multiplexed light amplifying basic units being connected in a multistage fashion and the second broadcast unit includes a plurality of distribution wavelength-multiplexed light amplifying basic units each identical in configuration to the first-stage wavelength-multiplexed light amplifying basic unit, with the distribution wavelength-multiplexed light amplifying basic units being connected in a multistage fashion and further includes a plurality of dropping optical filters for selectively outputting an optical signal with a predetermined wavelength to an output side of the optical amplifying section, while the selecting section includes switch sections, Y in number, for conducting switching between passage and interruption of the wavelength-multiplexed optical signal, coupling sections, Y in number, for conducting coupling between a wavelength-multiplexed optical signal line of the switching sections and an optical carrier signal line from the optical carrier signal outputting section and a multiplexing section for wavelength-multiplexing Y kinds of optical signals from the coupling sections to output a wavelength-multiplexed optical signal, and the switching sections are made to selectively conduct switching between an operation of allowing Y kinds of wavelength-multiplexed optical signals from the first broadcast unit to pass to be outputted from the multiplexing section and an operation of allowing Y kinds of optical carrier signals from the optical carrier signal outputting section to pass to be outputted from the multiplexing section.
Thus, the add-and-drop function is realizable with the same wavelength-multiplexed light amplifying units, which permits flexible extension and expansion of an optical network.
Moreover, in the wavelength-multiplexed light amplifying apparatus, it is also appropriate that an excitation light re-inputting section is provided in connection relation between the fourth wavelength-multiplexed light amplifying basic unit and at least the forefront wavelength-multiplexed light amplifying unit for removing a wavelength-multiplexed optical signal component from a leakage optical signal outputted from the fourth wavelength-multiplexed light amplifying basic unit to extract an excitation light and further for branching the extracted excitation light into a plurality of excitation lights to output the plurality of excitation lights, that an excitation light re-inputting section is provided between the forefront unit of the wavelength-multiplexed light amplifying base units connected in a multistage fashion and at least the forefront wavelength-multiplexed light amplifying unit for removing a wavelength-multiplexed optical signal component from a leakage optical signal outputted from the forefront wavelength-multiplexed light amplifying basic unit to extract an excitation light and further for branching the extracted excitation light into a plurality of excitation lights to output the plurality of excitation lights, or that provided are a fourth wavelength-multiplexed light amplifying basic unit identical in configuration to the wavelength-multiplexed light amplifying basic unit, with an output side of one optical amplifying section being connected to the signal input port of the wavelength-multiplexed light amplifying unit and an excitation light re-inputting section provided between the fourth wavelength-multiplexed light amplifying basic unit and the wavelength-multiplexed light amplifying basic unit for removing a wavelength-multiplexed optical signal component from a leakage optical signal outputted from the fourth wavelength-multiplexed light amplifying basic unit to extract an excitation light and further for branching the extracted excitation light into a plurality of excitation lights to output the plurality of excitation lights.
In addition, it is also appropriate that the excitation light re-inputting section includes a filter for removing a wavelength-multiplexed optical signal component from the leakage optical signal outputted from the fourth wavelength-multiplexed light amplifying basic unit to extract an excitation light and an optical branching section which is connected to the filter and an output side of which is connected to at least the signal input port of the second wavelength-multiplexed light amplifying basic unit in the forefront wavelength-multiplexed light amplifying unit and the signal input port of the third wavelength-multiplexed light amplifying basic unit, for putting the excitation light extracted by the filter in the forefront wavelength-multiplexed light amplifying unit, that the excitation light re-inputting section includes a filter for removing a wavelength-multiplexed optical signal component from the leakage optical signal outputted from the forefront wavelength-multiplexed light amplifying basic unit to extract an excitation light and an optical branching section which is connected to the filter and an output side of which is connected to at least the signal input port of the second wavelength-multiplexed light amplifying basic unit in the forefront wavelength-multiplexed light amplifying unit and the signal input port of the third wavelength-multiplexed light amplifying basic unit, for putting the excitation light extracted by the filter in the forefront wavelength-multiplexed light amplifying unit, or that the excitation light re-inputting section includes a filter for removing the wavelength-multiplexed optical signal component from a leakage optical signal outputted from the fourth wavelength-multiplexed light amplifying basic unit to extract an excitation light and an optical branching section which is connected to the filter and an output side of which is connected to the other input port of the wavelength-multiplexed light amplifying unit, for putting the excitation light extracted by the filter in the wavelength-multiplexed light amplifying basic unit.
Furthermore, in accordance with this invention, an optical add-and-drop apparatus using wavelength-multiplexed light amplifying basic units comprises a first stage unit for broadcasting a first wavelength-multiplexed optical signal composed from optical signals of X kinds of wavelengths, a first broadcast unit connected to an output side of the first-stage unit for broadcasting the first wavelength-multiplexed optical signal outputted from the first-stage unit, an additional signal outputting section for multiplexing a plurality of optical carrier signals to output a second wavelength-multiplexed optical signal, and a transmission outputting section connected to an output side of the first-stage unit and an output side of the additional signal outputting section for coupling the first wavelength-multiplexed optical signal from the first-stage unit with the second wavelength-multiplexed optical signal from the additional signal outputting section, wherein the first-stage unit is constructed as a first-stage wavelength-multiplexed light amplifying basic unit composed of an optical branching section having two input ports and two output ports for receiving the wavelength-multiplexed optical signal, composed from the optical signals of X kinds of wavelengths, through a signal input port forming one of the two input ports to output the first wavelength-multiplexed optical signal through the two output ports, with the other signal input port being connected to an excitation light source and optical amplifying sections respectively connected to the two output ports of the optical branching section, and the first broadcast unit includes a plurality of branching wavelength-multiplexed light amplifying basic units each identical in configuration to the first-stage wavelength-multiplexed light amplifying basic unit, with the plurality of branching wavelength-multiplexed light amplifying basic units being connected in a multistage fashion in a manner that the signal input port of the optical branching section in one multiplexing wavelength-multiplexed light amplifying basic unit is connected to an output side of the optical amplifying section in the preceding branching wavelength-multiplexed light amplifying basic unit, with one input port of the optical branching section in the forefront unit of the plurality of branching wavelength-multiplexed light amplifying basic units being connected to an output port of one optical amplifying section in the first-stage wavelength-multiplexed light amplifying basic unit, and with a plurality of dropping optical filters each for selecting the optical signal having a predetermined wavelength being provided at an output side of the optical amplifying section of the rearmost basic unit of the plurality of branching wavelength-multiplexed light amplifying basic units, and further the additional signal outputting section includes a plurality of optical carrier signal generating sections each for outputting an optical carrier signal and a multiplexing section connected to the side of the plurality of optical carrier signal generating sections for multiplexing the plurality of optical carrier signals from the plurality of optical carrier signal outputting sections to output the second wavelength-multiplexed optical signal, and even the transmission outputting section includes a coupling section connected to the first-stage unit and further to the additional signal outputting section for coupling the first wavelength-multiplexed optical signal from the first-stage unit with the second wavelength-multiplexed optical signal from the additional signal outputting section to output a third wavelength-multiplexed optical signal and an optical amplifier for amplifying the third wavelength-multiplexed optical signal from the coupling section to output the amplified third wavelength-multiplexed optical signal.
In addition, in the optical add-and-drop apparatus, it is also possible that further provided are an amplifying section provided on an input side of the first-stage unit for amplifying an optical signal to output an amplified optical signal and first dispersion compensating section connected to the amplifying section and further to the signal input port of the optical branching section in the first-stage unit for compensating for dispersion of the amplified optical signal to put a compensation optical signal subjected to the dispersion compensation in the signal port of the optical branching section.
Still additionally, in the optical add-and-drop apparatus, it is also possible that further provided is a second dispersion compensating section connected to the optical amplifier of the transmission outputting section for compensating for dispersion of the amplified third wavelength-multiplexed optical signal to put a compensation optical signal subjected to the dispersion compensation.
Moreover, in accordance with this invention, there is provided a wavelength-multiplexed light amplifying apparatus comprising an optical amplifying medium for amplifying input light, an optical branching/amplifying section for branching the optical signal, outputted from the optical amplifying medium, into a plurality of branched optical signals and further for amplifying each of the branched optical signals for compensating for a loss resulting from the branching, and gain control means connected to the optical branching/amplifying section for controlling a gain of the optical amplifying medium on the basis of the optical signal from the optical branching/amplifying section.
Thus, this method can conduct extraction processing for a desired wavelength in inconsideration of the influence on the other wavelengths and further can avoid, for example, the occurrence of coherent crosstalk easily. Additionally, leakage excitation light can be reused, thereby accomplishing efficient amplification.
Further, in accordance with this invention, there is provided an optical amplifier comprising a beam splitter splitting an input light, having a portion for amplifying
the input light, a pumping light source emitting a pumping light, a monitor monitoring an output light of the beam splitter, and a controller controlling a power of the pumping light in accordance with an output of the monitor.